The invention relates to observing sub-microscopic objects.
Optical microscopes are used to observe, e.g., microscopic variations on the surfaces of objects, including the surfaces of semiconductor materials onto which conductive or dielectric materials have been deposited. One type of optical microscope, a near-field scanning optical microscope (NSOM), uses an optical fiber with a fine tip to illuminate an object in the near-field, i.e., with the tip and the object separated by a distance less than the diameter of the tip. A photosensor then detects spatial variations in light transmitted through or reflected from the object, producing an output current in proportion to the intensity of the detected light. A rastering device, e.g., a scanning stage, is used to move the tip along the object and thus to generate an image illustrating the intensity of light across the surface area of the object.
The resolution of an image generated with an NSOM depends largely on the dimensions, geometry, and position of the tip. Resolution also depends on the size of the object and the distance between the tip and the object. Complex tip conditioning techniques have been developed to produce fibers with tips as small as tens of nanometers in diameter. However, such tips often must be processed or otherwise reconditioned before each scan of an object. Standard NSOMs also employ complex dithering techniques to ensure appropriate spacing between the fiber tip and the sensor surface.